VirtualBox is surprisingly easy to use for creating an OS-inside-an-OS, unless you have to move or back up your huge VDI machine files. You can avoid non-starting machines, however, with an XML file tweak.

Yatri Trivedi explains the moves needed to ensure a smooth transition, whether from one computer to another, or from your internal hard drive to an external. Here's what you need to do to your Virtualbox.xml file if you move things around:

If you're changing the location of your virtual machines, find the xml tags for "MachineEntry" and change the source path. If you're changing the location of your virtual disk images, find the xml tags for "HardDisk" and change the location there. As you can see, each machine is tied to a disk image by its UUID, which should not be changed. If you're moving virtual machines to a new computer, be sure to copy these this file over and replace the previous configuration file.

The whole process, and other smart moves, are detailed in screenshots at the How-To Geek's site.